The ability to perform parallel microanalysis on minute quantities of sample is important to the advancement of chemistry, biology, drug discovery and medicine. Today, the traditional 1536-well microtiter plate has been surpassed by microwell arrays which have an even greater number of reaction chambers and use lesser amounts of reagents due to efforts focused on maximizing time, throughput, and cost efficiencies. Although there are several types of microwell arrays available, fabrication used to generate high fidelity microfeatures having dimensions in tens of microns such as wells are frequently slow and expensive. Examples of common fabrication methods to produce these microfeatures include soft lithography, lithography, preferential etching of pre-existing arrays, milling, diamond machining, laser ablation, chaotropic etching and the like. However, all of these methods suffer from cost and capability limitations to varying degrees. Further, it is a special challenge to make articles with high density microfeatures that exhibit the desired optical features that are found in low-density microwell articles (e.g. 96-well plates). In addition, many microwell materials prove to be incompatible with the components of bioassays and chemical reactions and result in problems such as low sensitivity, high background signal, and lack of reproducibility. Thus, there continues to be a need for the development of improved microwell arrays.
Certain fiber optic bundles have been used to create microwell arrays. To act as an efficient waveguide, each fiber element must consist of a high refractive index core surrounded by a low refractive index cladding. Selective removal of the core glass by chemical etching to create a microwell lowers the refractive index mismatch when the original glass is replaced with a lower refractive index aqueous solution typically used in biological assays. This diminishes the waveguide characteristics of the fiber leading to increased light penetration through the cladding material. To overcome this problem light absorbing materials, for example certain metals, have been deposited as a thin layer on the interior sidewalls of the etched microwells. In addition to optical limitations, the fiber optic materials are often incompatible with many reaction conditions, particularly assays which are conducted in aqueous solutions and contain sensitive enzymatic reagents. Two major sources of incompatibility are the dissolution of the fiber optic substrate into the solution contained in the reaction chamber and the chemical reaction of the fiber optic substrate with assay components (e.g., proteins, nucleic acids) contained in the chamber. These chemical effects are exacerbated by the high surface to volume ratio in each microwell. These effects tend to degrade the performance of assays and reactions conducted in the fiber optic reaction chambers and frequently require additional processing to render the devices compatible with biological assays.
Due to technical difficulties in currently used processes for fabricating and/or coating arrays meeting these optical and chemical requirements, the range of assays that can be conducted in microwell arrays remains limited. Accordingly, there is a need for cost effective, high density microwell arrays that are compatible with a variety of assay and/or reaction conditions.